1. Field of the Invention
The present invention relates to an electric four-wheel drive vehicle in which front wheels are rotated by a driving force from an engine and rear wheels are rotated by a driving force from a motor, and also relates to an engine control unit for the electric four-wheel drive vehicle.
2. Description of the Related Art
Recently, vehicles running with motors used as driving sources have become increasingly popular. They are called eco-friendly cars and represented by electric vehicles and hybrid vehicles. As major features, each of those vehicles mounts a battery and has the function of utilizing electric power of the battery to generate torque from a motor for driving of the vehicle. In an electric vehicle, the battery is charged using an onboard or external charger. In a hybrid vehicle, the battery is charged by driving a generator with an engine (or causing a motor to generate electric power).
Along with the eco-friendly cars, an electric four-wheel drive (4WD) vehicle of the type directly driving front wheels by an engine and driving rear wheels by a motor has also recently become popular. In one known example of such an electric four-wheel drive vehicle, as disclosed in JP-A-2001-239852 (Patent Document 1), a dedicated generator is connected to an engine to generate electric power from the generator by utilizing a rotating force of the engine, and a DC (Direct Current) motor mounted for driving rear wheels is rotated by DC power outputted from the generator, thereby producing torque. That type of electric four-wheel drive vehicle provides a system that is superior in mountability to the known mechanical 4WD vehicle and is able to realize a lower cost because of advantages such as being batteryless. Also, the electric four-wheel drive vehicle equipped with the DC motor provides a very safe system in which electric power (DC power) generated by the generator is supplied to the DC motor directly (without power conversion). The electric four-wheel drive vehicle equipped with the DC motor is mainly applied to small-sized cars of 1-liter class from the viewpoint of mountability. The DC motor having a small output of about 2-4 kW is used in the small-sized car of 1-liter class because it has a small vehicle weight and operates the motor only in the take-off stage from start to a low speed.
As a known system analogous to the electric 4WD system, JP-A-2000-188804 (Patent Document 2), for example, discloses a hybrid vehicle in which a generator is mechanically connected to an engine, a large-capacity battery is connected to the generator, and a permanent-magnet synchronous motor for converting electric energy to motive power is connected to an output portion of the battery. In that hybrid vehicle, the generator generates electric power with a rotating force from the engine, and the synchronous motor is rotated by the generated electric power to produce the motive power. Further, because the battery is connected to an output portion of the generator, the electric power can be recovered to the battery during regenerative operation to apply an electric brake. That type of hybrid vehicle is mainly applied to large-sized cars of 2-liter class. The permanent-magnet synchronous motor having a large output of about 20 kW is used in the large-sized car of 2-liter class because it has a large vehicle weight and operates the motor over a wide speed range from start to a medium speed.